<!DOCTYPE html>
<html lang="en">
    <head>
        <meta charset="UTF-8" />
        <meta name="viewport" content="width=device-width, initial-scale=1.0" />
        <title>Document</title>
    </head>
    <body>
        <script>
            let arr = [1, 2, 3, 4];
            // let index1 = arr.findIndex((item, index, arr) => {
            //     return item == 3;
            // });
            // console.log(index1);
            //findeIndex
            Array.prototype.myFindIndex = function (callback) {
                for (let i = 0; i < this.length; i++) {
                    if (callback(this[i], i, this)) {
                        return i;
                    }
                }
                return -1;
            };

            let index1 = arr.myFindIndex((item, index, arr) => {
                return item == 3;
            });
            console.log(index1);
        </script>
    </body>
</html>
